The invention relates to sealing devices for rotating shafts where sealed fluid is employed to generate hydrostatic and hydrodynamic lift-off forces between stationary and rotating sealing elements, thereby effecting their separation and providing non-contact operation.
A sealing assembly of a non-contact type for rotating shafts is used in high speed and high pressure applications, where contacting type seals would experience overheating problems and failures caused by generation of excessive frictional heat. Contacting seals have pressure and speed limits depending primarily on whether the sealed fluid is liquid or gas. These limits are substantially lower with gas than with liquid, because as opposed to gas, liquid lubricates the opposed, rubbing surfaces of the sealing interface and can therefore expel a considerable amount of contact heat from said interface, hence permitting higher speeds and pressures.
Non-contact seals which are the subject to this invention, will also have speed and pressure limits. However, in the absence of contact, these limits are usually not because of frictional heat at the sealing interface, but moreover due to other factors such as material strength, viscous shear heat or permissible leakage value. The limits for non-contact seals are much higher than with contacting seals. Consequently, non-contact seals offer a preferred sealing solution for high speed, high pressure applications employed in centrifugal gas compressors, light-hydrocarbon pumps, boiler feed pumps, steam turbines and the like.
Non-contact seals are commonly more able to operate at elevated speeds and pressures regardless of whether the sealed fluid is a liquid, a gas or even a mixture of liquid and gas. Particularly, when sealed fluid change phase from gas to liquid and back, said seals offer an appreciable advantage. One of such non-contacting seals is of the face type, where one of the sealing surfaces is furnished with partial helical grooves as disclosed in U.S. Pat. Nos. 4,212,475, 3,704,019 or 3,499,653. This kind of seal has been applied to several sealing situations where not only high speeds and pressures were concerned but also in applications in which gas, liquid, or gas-liquid mixtures have been handled.
A disadvantage associated with sealing with non-contact seals is the effluvium which may be higher than the leakage expected when using a contacting seal in the same situation. This disadvantage becomes even more significant when the sealed fluid is either in liquid state of in a state of a liquid-gas mixture. This issue is associated with the fact that for the same volume of leakage, the density of liquid is several times higher than that of gas. Therefore the mass of amount leaked per unit of time will be much higher when leaking fluid is in liquid form rather than when it is in gaseous form. When sealing fluids at prominent pressure and speeds, the task is comparatively easier, if the sealed fluid is already in a gaseous state. If it is not and the sealed fluid is in liquid state, then there is always an inherent probability of high leakage.
From the above discussion, it could be concluded that vaporization at the seal faces of a contacting seal might offer a benefit since there would still be an abundance of liquid around the seal to entirely dissipate any frictional heat. However, in the prior art sealing arrangements it is not common to have the fluid change its phase from liquid to gas within the seal itself. As a matter of fact, gasification or vaporization at the sealing interface is though to be destructive to seal faces of liquid seals and it is therefore perpetually suppressed by the employment of flushing or cooling arrangements.
One such prior patent is U.S. Pat. No. 3,746,350 where a vortex type axial flow pumping device is employed to maintain an all liquid condition at the seal to extract frictional heat from the seal through liquid circulation. This heat removal lowers the temperature at the seal which then depresses the vapor pressure of the sealed liquid. Therewith, vapor pressure is kept safely below the pressure at the seal thus preventing liquid to vaporize. The pumping device operates by propelling liquid in an axial direction by vortex-forming threads shaped on the external surface of the rotatable part and on the internal surface of the surrounding non-rotatable part. The binary threads have opposite hands pending on direction of rotation, liquid will thereupon be urged in one of the two axial directions. Thread profile is optimized to achieve maximum flow rate of the liquid with given speeds of rotation.
A further prior patent is U.S. Pat. No. 4,243,230. Once more a pumping device is used to generate fluid pressure, which opposes loss of fluid from the housing during shaft rotation and which disengages the face seal to avoid loss of friction energy and to reduce wear. In this case, thread profile will not be optimized for maximum flow as in previously discussed patent, but instead will be optimized for maximum pressure differential toward the condition of zero or near zero flow, and this will normally result in a different thread profile.
In accordance with the invention, a seal arrangement is formed via combination of a non-contact seal and an axial flow pumping device. Said arrangement provides low-leakage performance of that of a gas seal even if sealed fluid is not a gas but rather a liquid or a gas-liquid mixture. This is accomplished by an axial flow pumping ring segment which is arranged to pump fluid away from the non-contact seal and back towards the source of said fluid. Thus without further replenishment of fluid flow through the axial flow pumping device will stall and a pressure drop is initiated. Subsequently, when fluid is stalled cooling is curbed and temperature of the fluid will rise. Both effects pressure drop and temperature rise cause vaporization of the fluid providing a non-contact gas seal with fluid in the preferred gaseous form for low leakage operation.
The prior patents discussed above present examples where pumping means inboard the sealing means are either employed to cool and circulate fluid or to seal, fluid and disengage a contacting seal. The invention exploits pumping means inboard sealing means to resolve the problem of high leakage on elevated pressure and speed seals for liquids where vaporization occurs within pumping means rather than having vaporization at the sealing faces which is oftentimes destructive. In that way, sealing means will encounter only gaseous vapor for low leakage operation.
The basic differences between this invention and the prior patents are:
As opposed to U.S. Pat. Nos. 4,212,475, 3,704,019 or 3,499,653 the present invention will result in low leakage regardless of whether seal fluid is liquid, gas or a mixture of both., whereas the above prior art will result in low leakage only if sealed fluid is a gas with liquid or liquid-gas mixture leakage will be higher.
This invention enhances vaporization by restricting circulation of pumped liquid to heat it and depressurize it. On the other hand, working with liquid only the seal of U.S. Pat. No. 3,746,350 suppresses vaporization by minimizing restriction to pumped liquid flow and channeling this flow through a cooling system and back to the seal.
The present invention uses a pressure drop optimized pumping device to vaporize the liquid while prior art uses pressure drop optimized pumping device to move a sealing subassembly in axial direction.
These and many other features and attendant advantages of the invention will become apparent as the invention becomes better understood by reference to the following detailed description when considered in conjunction with the accompanying drawings.